And then the dd.is_abstract_model() has a new API. In the
Meta class of a model which wants to be overrideable, you must
replace

abstract=dd.is_abstract_model('notes.Note')

by::

abstract = dd.is_abstract_model(__name__, ‘Note’)

The main reason for this change was a new feature: Lino now supports
multi-level plugin inheritance. We could now define a Person class
in lino_welfare.chatelet.lib.contacts which inherits
from lino_welfare.modlib.contacts which in turn inherits from
lino.modlib.contacts.